New advancements in projection systems utilize an optical semiconductor known as a digital micromirror device. A digital micrometer device chip may be the world's most sophisticated light switch. It contains an array from 750,000 to 1.3 million pivotally mounted microscopic mirrors. Each mirror many measure less than ⅕ of the width of a human hair and corresponds to one pixel in a projected image. The digital micromirror device chip can be combined with a digital video are graphic signal, a light source, and a projector lens so that the micromirrors reflect an all-digital image onto a screen or onto another surface.
Although there are variety of digital micromirror device configurations, typically micromirror are mounted on tiny hinges that enable each mirror to be tilted either toward the light source (on) in a projector system to reflect the light or away from the light source (off) creating a darker pixel on the projection surface. A bitstream to image code entering the semiconductor directs each mirror to switch on or off after several times per second. When the mirror is switched on more frequently than off the mirror reflects a light gray pixel. When the mirror is switched off more frequently than on the mirror reflects a darker gray pixel. Some projection systems can deflect pixels enough to generate 1024 shades of gray to convert the video are graphic signal entering the digital micromirror device into a highly detailed grayscale image. In some systems, light generated by a lamp passes through a color wheel as it travels to the surface of the digital micromirror device panel. The color wheel filters to light into red, green and blue. A single chip digital micromirror vice projector systems can create at least 16.7 million colors. When three digital micromirror device chips are utilized, more than 35 trillion colors can be produced. The on and off states of each micromirror are coordinated with the three basic building blocks of color, red, green and blue to produce a wide variety of colors.
Huibers et al U.S. Pat. No. 6,396,619 B1 discloses a deflectable spatial light modulator including a mirror plate that is substantially ridge and may be made up of a laminate having layers of silicon nitride and aluminum. In one embodiment, the mirror laminate may include a layer of aluminum sandwiched by two layers of silicon nitride. In other embodiments, include only a layer of aluminum and a layer of silicon nitride. Multi-layer arrangements with multiple layers of aluminum and/or silicon nitride are disclosed. The reference states that other materials besides aluminum (such as conductive and reflective metals) could be used. Other materials besides silicon nitride, such as silicon dioxide are also disclosed. The reference discloses that the silicon nitride layer may be 1400 Å thick and that the aluminum layer may be 700 Å thick. Disclosed also are one or more dielectric films, that act as a reflective coating, may be deposited on the mirror laminate to improve reflectivity.
A variety of digital micromirror devices (DMD) are known. FIG. 1 illustrates one embodiment of a prior art DMD that may be used in the present invention with the substitution of a unique mirror structure according to the present invention. As shown in FIG. 1, a DMD 10 may include a semiconductor device 12 such as a CMOS memory device that includes circuitry 13 that is used to activate an electrode(s) in response to a video or graphic signal. A first layer 14 is formed over the semiconductor device 12 and may include a yoke address electrode 16, and vias 18 formed therein down to the circuitry 13 on the semiconductor device 12, and a bias-reset bus 20. A second layer 22 is formed over the first layer 14 and may include a yoke 24 torsion hinge 26 and mirror address electrodes 28. A micromirror 32 is formed over the second layer 22 and positioned so that the micromirror 32 may be deflected diagonally when one of the electrodes 28 is activate by the semiconductor device 12. The micromirror include a reflective layer typically including aluminum. The DMD 10 shown in FIG. 1 while being an excellent engineering accomplishment is very complex, costly to manufacture and has low manufacturing yield. Further, the micromirror 32 may include defects as will be describe hereafter with respect to a second configuration of a DMD.
FIG. 2 illustrates a first subassembly 40 for a second type of DMD. The subassembly 40 may include a transparent layer 42 which may be any transparent material including, but not limited to, glass. A hinge 44 is formed on the transparent layer 44 and a micromirror 32 is secured thereto for pivotal movement with respect to the hinge 44 and the transparent layer 42.
FIG. 3 illustrates the first subassembly 40 including a plurality of micromirrors 32 each connected by a hinge 44 to the transparent layer 42. All of the component and subassemblies of the various DMD devices can be made by semiconductor or MEM micro processing techniques known to those skilled in the art.
FIG. 4 illustrates a second subassembly 46 of the second type of DMD and may include a semiconductor device 12 such as, but not limited to, a CMOS memory device. A plurality of electrodes 48, one for each micromirror 32 are formed over the semiconductor device 12 for communication with the circuitry (not shown) contained therein so that the electrode 48 may be selectively activated in response to a video or graphic signal.
FIG. 5 illustrates a DMD structure 10 that may be utilized by the present invention with the substitution of a unique micromirror according to the present invention. The DMD of FIG. 5 includes the first subassembly 40 flipped over and overlying the second subassembly 46 so the micromirrors 32 of the first subassembly 40 face and are closest to the electrodes 48 of the second subassembly 46. Spacers 50 are provided to position so that the micromirrors 32 are spaced a distance from the electrodes 48 and so that micromirror 32 is free to be defected or pivotally moved by the activation of an associated electrode 48. As illustrated in FIG. 5, when light is director on to the micromirrors 32, an electrode 48 associated with for each micromirror 32 may be activated cause the micromirror to pivotally move about the hinge 44. As a result, the light will be reflected or not depending on whether the electrode 48 associated with the micromirror 32 has be activated or not. As described above, depending on how fast and often a particular micromirror 32 is deflected by the electrode 48, the image projected by the micromirror 32 (pixel) will appear light or dark on the projection screen or other surface.
However, prior art micromirror structures often were troubled by the present of hillocks (raised features or bumps) 54 or voids 52 in the aluminum layer as shown in FIGS. 6 and 7. Typically the micromirror 32 include a sputtered on aluminum coating which may often include hillocks (raised features or bumps) 54 or voids 52. The hillocks 54 or voids 52 can cause artifacts or distortions in the projected image.
The present invention provides alternatives to and improvements over the micromirror, DMD and projection systems of the prior art.